Communications systems typically have some form of congestion management function to allocate the resources, such as bandwidth on a communications link, to users in a way that prevents the system from becoming congested. Typically some form of connection admission control (CAC) responds to new call requests and admits or refuses calls.
Integrated communications systems are now being developed which can carry a variety of types of traffic, such as voice, data and video. Traffic can be a mix of constant bit rate and variable bit rate traffic. Variable bit rate (VBR) traffic poses particular problems for traffic management as the bit rate of the traffic varies during a call. The effect of a number of VBR sources all operating together can result in a wide range of bit rates for the offered traffic. Clearly, if the offered traffic exceeds the available capacity, it cannot all be sent at the same time. Packet-based communications systems often use a cell-shaping function to “smooth” the peaks in the traffic to fit the traffic to the available resource, by buffering the traffic until it can be sent. However, delay-sensitive traffic such as voice cannot tolerate buffering and therefore cannot be treated in this way.
Some communications systems transport voice traffic using silence suppression, which suppresses silent periods in a conversation, thereby allowing the bandwidth which would have been occupied by data bits carrying those silent portions to be used by other traffic. The use of silence suppression provides some additional capacity on a communications link but also has the effect of turning the voice traffic into variable bit rate traffic. This makes it difficult to predict the actual sustained cell rate (average bandwidth) for a voice trunk using a fixed coding scheme because the ratio of talk to silence is not deterministic.
A network uses a policing or UPC function to ensure that traffic meets an agreed traffic contract. The traffic contract specifies parameters such as peak cell rate (PCR), and sustained cell rate (SCR). The ATM Forum defines a Generic Cell Rate Algorithm (GCRA) for ATM cell rate monitoring. The GCRA is used to define conformance with respect to the traffic contract. For each cell arrival, the GCRA determines whether the cell conforms to the traffic contract of the connection.
The GCRA can be a virtual scheduling algorithm or a continuous-state Leaky Bucket Algorithm. The GCRA is used to define, in an operational manner, the relationship between PCR and CDVT, and the relationship between SCR and the Burst Tolerance (BT). The BT can be derived from PCR, SCR, and MBS. In addition, the GCRA is used to specify the conformance, at the public or private UNI, of the declared values of the above two tolerances, as well as declared values of the traffic parameters PCR and SCR and MBS.
The GCRA algorithm operates on a cell by cell basis, which requires significant processing.